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January 31, 2015

January 31, 2015

Posted by **pyhsiclover** on Wednesday, May 22, 2013 at 2:11am.

Is your answer 0.57?

A 15 kg, 1m wide door which has frictionless hinges is closed but unlocked. A 400 g ball hits the exact middle of the door at a velocity of 35 m/s and bounces off elastically, thereby causing the door to slowly swing open. How long in seconds does it take for the door to fully open (rotate 90 degrees)?

- pyhsic question for count iblis -
**Count Iblis**, Wednesday, May 22, 2013 at 12:25pmThe answer I posted was correct as far as I can see, the problem may well be a typo in the problem itself, e.g. a 400 gram ball slamming into the door at the speed of a speeding car is not going to "slowly" open the door.

Anyway, it's a complete waste of time to e.g. try to see if what was meant is 3.5 m/s. I would suggest forgetting about this problem and moving on to another problem, you simply say that you did the problem correctly, that the teacher made a mistake and that you demand full marks for this problem.

- pyhsic question for count iblis -
**j**, Wednesday, May 22, 2013 at 8:33pmiblis got the problem right. for those trying to use jiskha to cheat on the brilliant problems, there was a mistake made in the acceptable answers for this week's question.

- pyhsic question for count iblis -
**roger**, Friday, May 24, 2013 at 10:02amibis ,I beg to differ,the door slowly open because the ball bounces back elastically,please don't be certain,when u are wrong

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